1. Field Of The Invention
The present invention relates generally to transmission of digital data through a communications link, and more particularly to a method of evaluating an incoming signal that is carrying digital data, in order to estimate the data transmission accuracy of this signal.
2. Description Of The Prior Art
Digital data transmission is a well-known and ubiquitous component of existing communication techniques. In the following text, the term "digital data transmission" will apply to the transmission of binary digital data, though such use does not exclude situations where the radix of transmitted numerals is greater than two. Following common usage in technical literature, the contraction "bit" will stand for "binary digit". Again, following common usage in the data communications field, the term "data symbol" will be used to stand for a pre-defined physical representation assigned to a given data bit such that data transmission can be physically executed through the communication channel. The physical form of the data symbol depends on the modulation technique employed, and can take the form of a voltage, current, optical intensity, or some other dimensioned physical parameter. In order to effect the transmission of multiple digital data bits, it is common to restrict each data symbol to ideally occupy a finite interval of time termed as a "bit period". A succession of data symbols injected into the communication channel by a suitable transmission mechanism constitutes the transmission of multiple data bits. It is common in the digital communications field to employ a nominally periodic "transmitter clock signal" in order to delineate the finite intervals of time to be occupied by each data symbol. The receiver of a digital data transmission link first receives, or extracts, the physical data symbols out of the communication channel. It is well-known that the physical characteristics of a channel can attenuate and distort the transmitted data symbols. Thus, the received data symbols are in general a degraded version of the corresponding transmitted data symbols. The receiver performs a "decision" operation whereby each received data symbol is mapped back to its corresponding data bit. The timing of the decision process is co-ordinated by a "receiver clock signal", often extracted from the incoming data symbols themselves by a well-known procedure known as "clock recovery". A common embodiment of the decision operation in the case of voltage symbols is to employ a clocked regenerative comparator circuit, sometimes referred to as a "decision circuit" or "flip-flop". Ideally, the decision operation occurs instantaneously at the instants of time specified by the receiver clock signal. The instantaneous decision process will be referred to as "sampling" of the data symbols.
An important step in achieving quality digital data transmission is detecting situations when the transmission is inaccurate. The inaccuracy of a transmission link is manifested in the form of bit errors, i.e., a transmitted bit being incorrectly identified at the receiver (including the catastrophic situation when none of the transmitted bits are properly identified at the receiver). Typical causes of bit errors in a digital data transmission link include the following:
1. Random additive white Gaussian noise (AWGN) distorts the amplitude of the data symbols and can cause independent bit errors when the signal-to-noise ratio becomes small.
2. Deterministic symbol degradations such as intersymbol interference (ISI), in conjunction with AWGN, can cause data pattern-dependent bit errors.
3. Timing jitter in the receiver clock signal can cause both independent as well as pattern-dependent bit errors.
The degradations described above can be aggravated by factors such as component aging, fluctuations in ambient operating conditions existing in the data transmission link, and catastrophic device degradation.
The primary performance metric for characterizing the occurrence of bit errors is the bit error rate (BER) of a link, defined as the relative frequency of bit errors over a given time interval for a given sequence of data bits. The maximum acceptable bit error rate (BER) of a digital data transmission link is usually a number much smaller than unity, such as 10.sup.-12. The BER measurement is typically defined for a test bit pattern such as a pseudo-random bit sequence (PRBS) of a particular periodicity. A BER of 10.sup.-12 means that on average, 1 bit error occurs for every 1 trillion consecutive data bits. It is important to note that depending on the statistics of the failure mechanism, there could be many intervals of 1 trillion bits where no errors occur, while many other such intervals contain more than one bit error. Hence, along with the BER, other metrics such as Error Free Seconds (EFS) and Severely Errored Seconds (SES) are employed to characterize a link.
Real-time performance monitoring of communication links allows incipient transmission link failures to be identified prior to their occurrence, and a timely responsive or corrective action instituted. Transmission techniques that are not transparent to the data bits typically employ a parity check or cyclic redundancy check (CRC) built into the transmission frame. For example, the widely employed Synchronous Optical Network (SONET) standard utilizes the Bit Interleaved Parity (BIP) technique. The parity violation rate at the receiver is an accurate measure of the BER over a wide range of BER values. It is also possible to monitor the BER by keeping track of violations of the line coding algorithm employed for symbol transmission.
In transmission systems known as data transparent links, the data bits are not accessed with reference to a particular transmission protocol, which precludes the use of parity violation, CRC, or code violation techniques. In transmissions of this type, the sequence of bits does not contain any standard, repeated sequences that can be relied on to evaluate the accuracy of transmission. In such systems, a method known as pseudo error monitoring can be used. In one such method an incoming stream of data symbols is sampled using at least two different methods, and the results are compared. If they agree for a particular data bit, that bit is assumed accurately received; if they disagree, the reception is assumed to be an error. An error rate calculated according to this method is termed a pseudo error rate (PER).
A number of prior art patents have addressed the above problem by generating one or more additional samplings of the incoming bit stream to compare with the main signal path data. U.S. Pat. No. 4,367,550 describes sampling each data bit at three separate instants of time during each bit period, once in the center for the main signal path data, and once on each side of center, i.e. once before and once after the center of the time period of the data bit. The method combines the two off-center samplings to generate data to compare with the main signal data. If the two agree, the bit is considered accurate; if they do not agree, it is considered a pseudo error, i.e., it is not known in fact that the main signal path data indication is in error, but it is highly questionable. This patent uses the data to determine a bit error rate (BER). This patent argues that a single, independent data point taken off-center yields an inaccurate result due to the slope of the bit pulse curve, and proposes that the aggregation/combination of the two samplings results in a pseudo error count that is much less sensitive to the precise offset of the samplings from the center. While U.S. Pat. No. 4,367,550 correctly identifies the sensitivity of the pseudo error rate to the precise offset in the timing instant, it fails to recognize that bit error rate curves (such as FIG. 2 of the patent) are usually plotted on a semi-logarithmic scale, i.e. X-axis linear, but Y-axis logarithmic. Thus, the Curve 3 of FIG. 2 in the patent will not possess the degree of "significantly lesser dependence on shifts of the scanning moment" as claimed. Further, in FIG. 2 of the patent, the two pseudo error rate curves corresponding to sampling instants B and C of FIG. 1 are ambiguously and questionably juxtaposed on the same X-axis. It is readily evident that the pseudo error rate curves will actually exhibit a variation with sampling instant as shown in FIG. 1 of the present disclosure. If in fact FIG. 2 of the patent exhibits the two curves offset in some manner along the X-axis, then the "combination" curve combines data from different portions of the bit period. This procedure is questionable since, in general, the received data symbols need not be symmetric about a vertical line passing through the center of the bit period. It should be also noted that the data bits produced by sampling the main signal path are used as the reference for judging the veracity of the bits generated by off-center sampling. Hence, any posited relationship between the pseudo error rate and the inherent BER of the main signal path is insubstantial by definition.
U.S. Pat. No. 5,333,147 describes a method for counting the number of times a signal invades predetermined areas on a digital display, giving an indication of a pseudo error rate. This method has the disadvantage of being very complex to implement. U.S. Pat. No. 3,721,959 defines a rectangular region in what is known as an "eye diagram." An eye diagram is an amplitude vs time display of the incoming data signal, with both positive and negative signals superimposed to give an appearance of an eye. Ideally, this pattern would be rectangular, but due to dispersion and losses in signal transmission it is degraded, causing a reduced amplitude and rounded corners. According to the method, a rectangle inside the eye diagram is defined, and a count is made of the number of times the signal invades the rectangle. This count is used as an indication of the error rate. Again, this technique is complex to implement, requiring a large sampling rate on the principle of a real-time oscilloscope.
In view of the above described prior art methods, it is apparent that there is a need for a more meaningful and simple method of detecting a faulty signal.